1. Field of the Invention
The present invention relates to a method of forming a metal plate on a semiconductor wafer, and more particularly, to a method of forming a metal plate of a fingerprint sensor chip on a semiconductor wafer.
2. Description of the Prior Art
The biometric sensor chip used for the prior art fingerprint detector is a semiconductor product. The biometric chip comprises approximately ninety thousand metal sensor plates arrayed in a 300.times.300 pixel matrix. This matrix is sandwiched between an inter-metal dielectric (IMD) and a passivation layer, and is used as the sensor array of the fingerprint detector.
When the finger of the user touches the passivation layer of the biometric sensor chip, each sensor plate in the sensor array detects the static voltage in its area relative to the finger. The changing topography of the finger causes relative changes in voltage in each sensor plate across the sensor array. This pattern of relative voltages generates an image of the fingerprint, which can be passed on to external circuitry for recognition.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a layout schematic diagram of a sensor plate 22 of the biometric sensor array of the prior art. FIG. 2 is a cross-sectional view along line 2--2 of the biometric sensor chip shown in FIG. 1. A prior art biometric sensor array comprises ninety thousand metal sensor plates 22, and each sensor plate 22 is formed on the surface of a semiconductor wafer 10. The semiconductor wafer 10 comprises a dielectric layer 12, a metallic line layer made of several interconnection lines 14 positioned on the dielectric layer 12, an IMD 15 covering the interconnection lines 14 and the dielectric layer 12, a metal plate positioned on the IMD 15 to be a sensor plate 22, and a passiviation layer 24 covering the sensor plate 22 and the IMD 15 to protect the circuitry on the semiconductor wafer 10. The IMD 15 is a compound structure which comprises spin on glass (SOG) 18 and a silicon oxide layer 20.
In the formation of the prior art biometric sensor chip, a dielectric layer 12 is deposited on the surface of the semiconductor wafer 10, then a first metallic layer is formed on the dielectric layer 12 and is processed into the interconnection lines 14 using photolithography and etching. These interconnection lines 14 electrically connect the sensor array to the image recognition circuit of the biometric sensor chip, or to other internal circuitry. An SOG 18 is formed on the interconnection lines 14 and the dielectric layer 12 to form a spacer on both sides of each interconnection line 14 which smoothes the corners between each interconnection line 14 and the dielectric layer 12. Next, a silicon oxide layer 20 is formed on the interconnection lines 14 and the dielectric layer 12 to prevent the interconnection lines 14 from being corroded by subsequent processes, and which completes the formation of the IMD 15 compound structure.
After the IMD 15 is formed on the semiconductor wafer 10, a second metallic layer is formed on the silicon oxide layer 20 which also undergoes photolithography and etching to form the sensor plates 22. Finally, a passiviation layer 24 is formed on the sensor plates 22 and the IMD 15 to protect the circuitry on the semiconductor wafer 10. When the finger of the user touches the passivation layer 24, each sensor plate 22 senses the static voltage in its area relative to the finger, and all ninety thousand sensor plates 22 together make a relative fingerprint image.
However, the area between each interconnection line 14 is very uneven with the SOG spacers 18 and the interconnection lines themselves. Consequently, the silicon oxide 20, which is of uniform thickness and which is formed across this uneven surface, is not a level surface. When the metal sensor plates 22 are formed on this uneven silicon oxide layer 20, they become distorted, with raised edges and a sunken center. Such non-planar metal sensor plates adversely affect the sensitivity and accuracy of the entire sensor chip as the distance from an individual sensor plate to the user's finger is not consistent across the area of the sensor plate.
Furthermore, the uneven topography of the sensor plates 22 causes the passivation layer 24 to be uneven. This is especially true when the passivation layer 24 is deposited on the raised edges of the sensor plates 22, which causes peaks and valleys to form in the passivation layer 24. Consequently, when the biometric sensor chip is mounted on a printed circuit board (PCB) and it undergoes a cleaning process, the high-pressure water used in cleaning will strike the slopes of the passivation layer 24 and generate cracks. Water droplets will then be able to infiltrate down into the interconnection layer through these cracks and cause short-circuiting. The cracks also weaken the passivation layer 24 of the biometric sensor chip, making it more susceptible to damage during packaging and actual use.